Electron emitters



Patented May 2, 1950 ELECTRON EMITTERS Imre Patai, Washington, 1). 0.; 11.: Russell'Bishop', administrator of said Imre Patai, deceased No Drawing. Application August 28, 1947, Serial No. 771,139

1 Claim.

This invention relates to electron emitters and to their method of manufacture. More particularly this invention is concerned with an improved method of manufacture of electron emitting cathodes of the oxide-coated type used in electronic and ionic discharge devices.

In prior methods of manufacture of oxidecoated electron emitting cathodes, a core metal of nickel, alloys of nickel, and other metals are coated with one or more layers of active metal, e. g., barium and/or strontium carbonates by spraying, dipping or dragging the core through a mixture of finely ground carbonates suspended in water, for example, with or without a suitable binder. Also, it has been proposed to coat the core with the carbonatesby electropharesis to obtain a more uniform structure and thickness of the coating. After the application of the carbonate coating, the cathode is mounted in the discharge tube containing other electrodes, especially an anode. The tube is then evacuated and the cathode is heated electrically to a temperature of about 1400" K. to reduce the carbonates to-o'xides. Thereafter, a voltage is applied between an anode and the cathode, at reduced temperature, until emission builds up to its normal value. This latter action is referred to as the activation process, and is effective in forming free metals throughout the oxides, which metals diffuse to the surface of the cathode where they form a very thin, perhaps monatomic, layer which is the seat of the electron emission.

During operation of the oxide-coated cathode free barium, and/r strontium constantly is being evaporated from the surface. This evaporated barium and/ or strontium is replaced, however, by the available supply of barium and/or strontium oxide in the coating which is reduced to furnish the free metal during the active life of the emitter. It is important therefore that a good supply of the oxides of barium and/or strontium be available.

Ithas been impracticable to acquire a good supply of the alkaline earths by providing a thick coating of the oxide layer, because the oxide layer is non-conducting and any material increase in thickness of the coating increases also the crossresistance for the emission current. In order to obviate this disadvantage it has been proposed to modify the cathode coatings by admixing repository metallic particles, e. g., aluminum and zirconium oxides with the electron-emissive materials employed, which metal particles serve to reduce the cross-resistance of the coating and serve also as a metallic reducing agent for the alkaline earth oxides. Cathodes containing these added metals are referred to in the art as combined cathodes, which by the following definition comprises a metal carrier or core of tungsten, platinum, platinum alloy, nickel or nickel alloy coated with barium and strontium oxides (or other compounds which can easily decompose by heat'to the oxides) through which finely divided core material or its equivalent is dispersed; and is characterized by long life attributed to the presence of the finely divided core material in the oxide coating which acts to increase the rate of dissociation of the oxides to provide a reservoir for the activematerial.

In so far as the processes used heretofore contemplate applying the coating in the form of metallic powder, the results obtained have not been entirely satisfactory because it is very difficult to'produce a fine powder of pure metals; and the degree and character of their dispersion in the active metal have failed to be such as sensibly to improve the quality of the resulting coat- It is an object of this invention to increase the operating life of oxide coated electron emitters.

It is a further object to provide an improved method of manufacture of an oxide coated cathode wherein finely pulvenized repository reducing metals having the desired final surface configuration may be mixed with the active oxide of the coating to provide an emitter having characteristics of life and thermionic emission at least equal to the combined oxide-coated cathodes manufactured by the vapor process as taught by Hertz (U. S. Patent 1,720,675), without the concomitant disadvantages associated with that method.

I have discovered that powders of the desired reducing metals, e. g., copper, nickel, tungsten, cobalt, etc., may be prepared in colloidal fineness by grinding in a ball mill, for example, the compounds of these metals, especially the oxides or compounds which, when heated in vacuum, yield oxides.

As a result of this discovery I am able to prepare a fine dispersion, preferably a colloidal solution, of a mixture of alkaline earth carbonates or oxalates and oxides or other compounds of the reducing metal. These fine powders, preferably in colloidal solution, are cataphoretically deposited on the surface of the core material by passing a current through the solution and the metallic core. I have found that by employing the compounds of both the alkaline earths and the compounds of the reducing metal simultaneous cataphoretic deposition is possible since the compounds of the reducing metal may be selected which have migration properties similar to the compounds of the alkali earth metals; i. e., most metal oxides, carbonates, oxalates, etc., migrate toward the cathode during electrophoresis, whereas pure metals mostly migrate toward'the anode.

In a preferred embodiment of the invention barium and strontium carbonates in equal proportion are ground in a ball mill to a particle size of about 0.2 to 5 microns, and a compound of the reducing metal, e. g., copper oxide, is ground also in a ball mill to similar particle size and added in suitable ratio (2 to 8% depending upon the particular metal selected) to powders of barium and strontium compounds. These powders are then dispersed in a liquid such as butyl alcohol, acetone, or the like. This fine, preferably colloidal, dispersion is then cataphoretically deposited on the surface of the core material by passing a current through the solution and the metallic core. After the cataphoresis has been carried out for a time sufiicient to obtain a coating on the core of suflicient thickness, the core is dried and heated (to about 100-800 K.) in an atmosphere, such as hydrogen, to reduce the copper oxide to metallic copper. It is important that during this reduction of the added metal (reducing metal) the temperature be kept lower than the dissociation temperature of the alkaline earth carbonates.

The cathode is now ready for mounting in the discharge tube for converting of the carbonates to oxides; and for the application of the activating process. To reduce the carbonates to oxides, the tube is evacuated and the cathode is heated electrically to a temperature of about l400 K. Thereafter to activate the coating a voltage is applied between the anode and the cathode at reduced temperatures, until the emission builds up. If metals other than copper oxide, for example the oxides, carbonates or oxalates of cobalt or nickel, are used, it is useful to repeat the heating in a reducing atmosphere after the decomposition of alkaline earth carbonates.

Obviously many modifications and variations of the invention as hereinbefore described may be made without departing from the spirit and scope thereof, and only such limitations should be imposed as are indicated in the appended claim.

I claim:

The method of directly activating a combined oxide-coated thermionic cathode for the emission of electrons comprising preparing a colloidal dispersion of a mixture of finely divided powders of an oxide of a reducing metal selected from the group of metals consisting of copper, nickel, tungsten, and. cobalt, and an alkaline earth carbonate, coating a metal core with the mixture of said dispersed particles by passing an electric current through the colloidal dispersion of said mixture and said metal core, heating said coated core in a hydrogen atmosphere to eifect a reduction of the oxide of said reducing metal, mounting said coated core in an envelope, heating said coated core to a temperature of about 1400 K. to efifect the reduction of said alkaline earth carbonate to an oxide and thereafter activating said coating by subjecting said cathode to a positive electric field while maintaining said cathode at a reduced temperature.

IMRE PATAI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,897,902 Harsanyi Feb. 14, 1933 2,041,802 Wilson et a1 May 26, 1936 2,333,710 Deimel Nov. 9, 1943 OTHER REFERENCES Bidgood et al., cataphoresis and Alundum Coatings, trans. Electro. Soc, vol. 8'7, pp. 30, 34. 

